1. Field of the Invention
The invention relates generally to navigation guidance systems, and more particularly, to such systems employing equipment for air derivation of guidance data from signals received from the commutated array of a Doppler simulating ground beacon system.
2. Description of the Prior Art
The invention relates to a radio navigation receiver for use in a known type of Doppler radio navigation system wherein there is a ground beacon transmission format involving commutation of a first radio frequency in turn to a linear array of radiators (typically on the order of 60.lambda. in electrical length) so as to simulate constant velocity unidirectional or bidirectional scanning of the array. In such systems, a main bearing signal is transmitted which, as "seen" by a receiver of the system, undergoes a Doppler frequency shift proportional to the sine of the angle subtended by the receiver normal to the axis of the array. The transmission format also includes simultaneous transmission of a reference signal of a second radio frequency (offset from the first), whereby the receiver is able to utilize this and the main signal to derive a Doppler beat waveform having a frequency indicative of the said angle.
A form of beacon having the above defined transmission format, with unidirectional scanning, is described in our British Pat. No. 1,225,190, and a form of beacon having the above defined transmission format with bidirectional scanning is described in British Pat. No. 1,234,541.
A feature of the above described transmission format is that, in an elevation guidance system wherein the commutated array is vertical, the direct path signal as received by a radio receiver of the system may in principle be readily distinguished from a multipath signal, i.e., a signal which has been reflected from the ground (a multipath signal), for example, because the Doppler frequency shift imparted to the direct path signal is in the opposite direction to that imparted to the ground reflected signal. Accordingly, the receiver fundamentally needs to include a wideband Doppler information filter having a passband covering the range of Doppler beat, frequencies corresponding to the direct path signal, but having a rejection band covering such frequencies corresponding to multipath signals.
Typical beat frequencies for a Doppler elevation system are (for direct path signals) 14.88 KHz at 0.degree. elevation down to 8.22 KHz at 10.degree. elevation. The ground multipath signal may be typically between 14.88 KHz (0.degree.) and 21.54 KHz (-10.degree.). The information filter therefore would have a passband of approximately 8.2 to 14.8 KHz.
It is known, however, that the frequency side-lobes of the reflected signals spread more or less uniformly across the passband of the filter, producing significant bearing indication error for flight paths over the whole sector. A zero-crossing detector and counter are used to register the total number of counts occurring over a complete digitization period for actual data extraction. That instrumentation is known and is described in British Pat. No. 1,234,541.
The concept of digitization relates to incrementing of the radio frequency phase of the energy fed to the ground beacon commutated array over a "data slot" (i.e., scans utilized in obtaining a single data point). The concept is described in more detail in U.S. Pat. application Ser. No. 480,006 filed June 17, 1974, entitled "Doppler Microwave Landing System Signal Simulator".
A further error occurs, due to frequency pulling of the beat signal toward the center frequency of the filter. This characteristic is well known and is referred to as filter truncation. These two effects are essentially independent of one another, and combine linearly in respect to the errors they generate. The manner in which the present invention deals with these prior art problems will be understood as this description proceeds.